Tinker, Tailor, Soldier, Spy

So EXP axles are for advanced builders for use in "multi-techs", which are techs which are more or less one body, but have multiple different segments which the game identifies as individual techs.

The way Terratech works, you can't have any attachment points (the red dots) which actually move with respect to the cab, so things like giant turrets, or joints in the middle of the tech, bulldozer bucket arms, or even extendable pieces aren't possible by using only a single tech.

There is a way around this, though, but prepare for a challenge. You can use multiple techs to accomplish these things, where each tech is it's own moving segment. This is what the axles are for. You attach the axle to one tech, and the bearing to another tech, then intersect them very carefully. The game identifies the techs (and renders the physics) independently, but if you've built it right, it will work correctly.

Here's an example of such a tech (this is NOT my tech, just the first example I found on steam. Credit to xDawnshadow). It uses an entirely separate tech for the turret versus the base, but the effect looks like a giant tank turret. (If you load it, press X before use to separate the turret from the base)

Multi-techs aren't well supported in the game, right now unfortunately. There's a lot of simple things which just don't work because one tech doesn't consider the other, and when blocks intersect, the physics solves this by ejecting one side (breaking your multi-tech). Still it's possible with patience, and some of the coolest big techs are multitechs, such as the video below (also not mine).

The Pancat ( ͡° ͜ʖ ͡°)

I will anwser your question in Human language: first you need a cab, then you need to activate the build beam, move the cab closer to the axle (that you have presumabily placed on the other tech), and place the second part of that axle on the cab